This invention relates to an optical system for various apparatuses and measuring instruments with a semiconductor laser as the light source, the characteristic feature of which resides in providing the most favorable optical system without necessity for correcting, by any optical system, an astigmatic difference (hereinafter simply called "As") which arises unavoidably when the semiconductor laser is used as the light source.
In general, projecting light beams from a semiconductor laser are governed by a direction, in which their divergent angles orthogonally intersect. Also, the original point of divergence differs depending on the abovementioned orthogonally intersecting direction. This is ascribable to the internal construction of the semiconductor laser per se, and to the fact that a region of the light emitting portion does not take a circular form as in the gas laser, but a rectangular form.
FIGS. 1A and 1B show a state of divergence of a beam from a semiconductor laser, wherein FIG. 1A is a top plan view of the semiconductor laser, and FIG. 1B is a side elevational view thereof. In the drawing, a reference numeral 1 designates a semiconductor laser chip, and a numeral 2 refers to a junction surface thereof. A numeral 4 refers to the original divergent point of the light beam in a direction parallel to this junction surface (hereinafter referred to as "horizontal direction"), while a numeral 5 refers to the original divergent point of the beam in a direction vertical to the junction surface (hereinafter referred to as "vertical direction"). The original divergent point 4 in the horizontal direction is positioned off the beam projecting surface, while the original divergent point 5 in the vertical direction is positioned in the vicinity of the beam projecting surface.
When a focussed spot of the projecting light from the semiconductor laser having such As is formed by use of an ordinary lens (an anamorphic lens system including a cylindrical lens may serve the purpose; this designates any ordinary lens which requires no correction of As), the focussed spot does not form a beam waist on the image focussing plane in the cross-section thereof in both vertical and horizontal directions.
FIGS. 2A, 2B and 2C explain the above situation, in which FIG. 2A is a top plan view, and FIG. 2B is a side elevational view. In more detail, when a light beam from the semiconductor laser is focussed by means of a lens 6, the shape of the focussed spot changes by varying the image forming plane. FIG. 2C shows in (I), (II), (III) and (IV) the changes in the shape of every focussed spot when the image forming plane is varied, from which it is seen that the beam waist in the horizontal direction exists in the position of (II) and the beam waist in the vertical direction is present at the position of (IV).
As one example of correcting this difference in position of the beam waist in both vertical and horizontal directions, laid-open Japanese patent application No. 52-24542 discloses that the original divergent points in the orthogonally intersecting directions are caused to meet each other by use of cylindrical lenses, each having a different radius of curvature and the generatrices of which orthogonally intersect mutually.
Such method of correcting the original point of beam diversion with an optical system (which will be called "correction of As") is capable of regularizing the beam waist position on the image forming plane, and of forming as small a focussed spot as possible. This is the requisite condition even in the case of using the focussed beam spot in an interference experiment by simple collimation, but not intending to obtain such small focussed spot.
The correction of As, however, is extremely difficult to accomplish, since the optical system should be adjusted very precisely. In other words, in order to converge a light beam having different original divergent points in the two mutually orthogonally intersecting directions at one specific point, the same adjustment of the optical system must be done twice with respect to each of these orthogonally intersecting two directions, which is very troublesome and time-taking. Moreover, there is no assurance that such adjustment can be successfully done independently for each direction. There might possibly take place such a situation that, when one of the two directions is adjusted, the other gets out of order. Consequently, for the successful adjustment of the optical system, skills of veteran technicians are indispensable.
Furthermore, the quantity of such As is not constant, but it differs from a semiconductor laser to the other. Not to say of the laser having different structures, even the lasers of identical structure possess varying quantity of As depending on their production lot. Accordingly, correction of such As with an optical system necessitates use of a differing optical system for each of the semiconductor lasers, or provision of an adjusting mechanism with the consequence that burden would increase on the part of the optical system to be used, which reflects on the operational cost and complexity in such adjusting operation. Further, even with the same kind of laser, there occurs such a situation that the quantity of As varies depending on current value.
On the other hand, when the semiconductor laser is used for image recording, display, and so forth, there is no necessity for regularizing the original divergent points of light beam, unlike the case of using the light beam for an interference test. This is only a problem relative to the shape and peak power. Therefore, if the required specifications are satisfied when the image recording and display are performed, the correction of As becomes unnecessary.